Ok, lets think about this th US had the M1 garand, M1 carbine, BAR ,and Thompson. The Garand was the only semi-auto rifle that saw any real service in WW2. The carbine was a great compromise between full-pwoer rifles and pistols. The BAR was probaly the third-worst squad-level MG in WW2, beating only Italian and Japanese designs. It was magazine-faed(low capacity) and had the mag on the bottom (slow reload). However, it was very mobile and didn't need a sqaud to haul the ammo. The Thompson, aside from the $$$ issue and weight (11lb), was near-perfect.
The British had the Lee-Enfiled No. 4, the Sten gun, and the Bren gun. The No. 4 was the best bolt gun of the war, with a large magazine and a high ROF (for a bolt-action gun) The Sten has a reputation as one of the worst guns ever, though it wasn't. It worked and was there, in fairly large numbers. The Bren gunwas top-fed, and the only complaint I've heard was that it was too acurate for supresive fire.
The Germans had the MP38/MP40, the Kar98k, and the MG42. THe Mp40 was good, though hoplessly under-issued for most units. The kar98's only failing was that it was semi-obsolete. It was slow-firing bolt gun in a world dominaded by M1s, no. 4s and the PPSh-41. The MG 42 was heavy and a nightmare-fore allied infantry and German quartermasters. its high ROF ate up ammo and barrels very fast.
The Russians had the Mosin-Nagant, the PPSh-41, and the DP. The Mosin was little more than a sidenote by the end, though it gave good service. The PPSh-41 was the greatest SMG of the war, as it was Mass-produced in huge numbers. With its 71 round drum and 900prm fire rate, it was well-like. The DP wa s typicaly russian, with crude but sturdy engineering and a top-mounted pan magazine. It was easy to change but hard to carry a lot of, IMHO.
I'll adress the :wonder weapons in another post, this one in long enough